The first task of this project is to recruit human subjects required for the Program Project. The second is to audiologically characterize representative samples of human of young and old subjects with normal hearing and hearing losses who will undergo comparative experiments in psychoacoustics, evoked potentials, reflex behavior, and positron emission tomography. The third aim is to evaluate age-related changes in the psychophysics of temporal processing, the processing of signal in noise, and binaural processing in humans. Initial gap detection studies have demonstrated age-related slowing in auditory temporal processing in older adults with essentially normal peripheral sensitivity. Also, we have shown that processing speed decreases disproportionately as the complexity of the stimulus configuration increases. This suggests that age-related changes occur at multiple sites throughout the central auditory nervous system and that additional, disproportionately large, decrements in processing will be revealed with binaural processing is required. Our fourth aim is to utilize Positron Emission Tomography (PET)) to investigate the functional changes that occur in the central auditory system as a result of aging and presbycusis. We propose a detailed series of experiments to separate the effects of aging from high-frequency sensorineural hearing loss (SNHL). In addition, we plan to investigate the functional neuroanatomy of abnormally rapid loudness growth ("recruitment") and abnormal sound localization, two impairments which are associated with age and SNHL. We will do so by determining the relationship between the intensity of an external tone and the degree of activation in the auditory cortex and how this relationship is affected by aging and by age-related SNHL. We will pursue the question of how high- frequency SNHL affects the cortical frequency-place map in old subjects. And we will conduct experiments to determine how binaural sound cues involved in sound localization are processed by the cerebral cortex and to determine the effect aging and high-frequency SNHL have on binaural processing.